It is generally recognized that the fabrics comprising synthetic polymer fibers, such as polyethylene terephthalate (PET) fibers, and blends of natural and synthetic fibers have an undesirable propensity upon prolonged use to exhibit small, compact groupings of entangled fibers (i.e., fuzzballs) on the fabric surface. Such fiber groupings commonly are termed "pills" and tend to form and to tenaciously adhere to the surface of the fabric as the fabric encounters surface abrasion during normal use. The aesthetic appearance of fabric accordingly may be adversely influenced by these relatively compact groupings of entangled fibers which are retained on the surface of the fabrics.
Heretofore, it has been believed that such pills can be traced to the relatively high strength of the synthetic fibers present in the fabric. For instance, the pills may be more or less permanently attached to the fabric surface by one or a few synthetic polymer fibers extending out of the fabric which will resist breakage as surface abrasion continues. This theory of pill formation is supported by the lower level of the retention of undesired fuzzballs on the surface of the fabrics consisting solely of cotton fibers following the same surface abrasion conditions. It is believed, for instance, that any entangled cotton fibers which form at the surface of the fabric readily break away since the cotton fibers are of an inherently lesser strength.
This pilling problem may be observed in fabrics formed in whole or in part from polyethylene terephthalate fibers which are either in a staple or in a continuous configuration. Pills commonly are observed on the fabric formed from blends of cotton and staple polyethylene terephthalate fibers following extended use. While the pills may be observed in fabrics having a wide variety of constructions, they are frequently observed on loosely constructed fabrics, particularly knitted fabrics which comprise polyethylene terephthalate fibers.
One approach heretofore proposed to reduce the tenacity of the polyethylene terephthalate fibers to pill initially to limit the molecular weight (measured as the intrinsic viscosity) of the polyethylene terephthalate polymer prior to fiber formation. Such reduced intrinsic viscosity provides a general indication of reduced polymeric chain length and leads to fibers having a lesser strength. Accordingly, when such entangled fibers become free on the surface of the fabric following abrasion, they tend to cleanly break away more readily and do not serve as a secure link which retains a fuzzball at the surface of the fabric. It has been found, however, that when this approach is followed, it is more difficult initially to form and process the polyethylene terephthalate into a fabric using standard processing conditions because of their reduced strength. Such reduced strength can lead to premature fiber breakage and to concomitant processing disadvantages.
In U.S. Pat. Nos. 3,104,450; 3,576,773; 3,580,874; 3,607,804; 3,991,035; and 4,004,878, the incorporation of certain moieties in the backbone of the polyethylene terephthalate fibers, to reduce the fibers' strength, is proposed in an effort to improve the pilling characteristics.
Another approach is to weaken the polyester fibers after forming. In U.S. Pat. Nos. 3,834,868, 4,270,913 and 4,666,454 the degradation of previously formed polyethylene terephthalate fibers is proposed in an effort to reduce the propensity of the fiber to pill.
U.S. Pat. No. 4,666,454 discloses a fabric containing a polyethylene terephthalate fibers having a reduced tendency to pill. The disclosed PET fibers include 5 to 15% by weight of monomer polyethylene glycol (PEG) with an average molecular weight of 200 to 2000. Other moieties, e.g. diacids such as isophthalic acid, adipic acid and sulfoisophthalic acid, may be added at the rate of 1 to 10% by weight of monomer, but only so long as those moieties "do not interfere" with the fabric's ability to have a reduced tendency to pill. The disclosed PET fibers are subsequently treated in a dye bath in which the fabric is both colored and the intrinsic viscosity of the PET fibers is reduced. The dye bath conditions are as follows: a pH of 3 to 5; temperature of 90.degree. to 160.degree. C. ; residence time of 10 to 90 minutes. The reduced tendency to pill is made possible by the presence of degradation susceptible units derived from polyethylene glycol. The resulting fabric when subjected to prolonged surface abrasion exhibits no substantial tendency to pill.
In the fabric dying arts, it is known that cationic dyeable fabrics will weaken when subjected to the dye bath. Moreover, it is known that such weakening will be arrested by the addition of salts, i.e. Glauber's salts, to the dye bath. Yet, no one has ever proposed that the pilling properties of fabric may be controlled by arresting the weakening of a fabric by the addition of a salt.
These prior solutions, however, have not led to commercially attractive processes. This failure is due to the fact that it is difficult to implement those processes. For example, the former approach, weakening the polymer before processing, causes the fibers to break prematurely, while the latter approaches are difficult to control.